Types of precipitation
1. Rain (RA) - Rain is liquid precipitation that reaches the surface in the form of drops that are greater
than 0.5 millimeters in diameter. The intensity of rain is determined by the accumulation over a given
time. Categories of rain are light, moderate and heavy.
2. Snow (SN) - Snow is an aggregate of ice crystals that form into flakes. Snow forms at temperatures
below freezing. For snow to reach the earth's surface the entire temperature profile in the troposphere
needs to be at or below freezing. It can be slightly above freezing in some layers if the layer is not
warm or deep enough the melt the snow flakes much. The intensity of snow is determined by the
accumulation over a given time. Categories of snow are light, moderate and heavy.
3. Snow Pellets (GS) - A snow pellet is precipitation that grows by supercooled water accreting on ice
crystals or snow flakes. Snow pellets can also occur when a snowflake melts about half way then
refreezes as it falls. Snow pellets have characteristics of hail, sleet and snow. With sleet (ice pellets),
the snowflake almost completely melts before refreezing thus sleet has a hard ice appearance. Soft hail
grows in the same way snow pellets can grow and that is ice crystals and supercooled water accreting
on the surface. Snow pellets will crush and break apart when pressed. They can bounce off objects like
sleet does. Snow pellets have a whiter appearance than sleet. Snow pellets have small air pockets
embedded within their structure and have visual remnants of ice crystals unlike sleet. Snow pellets are
typically a couple to several millimeters in size.
4. Snow Grains (SG) - Snow grains are small grains of ice. They do not produce much accumulation
and are the solid equivalent to drizzle.
5. Ice Crystals (IC) - Also called diamond dust. They are small ice crystals that float with the wind.
6. Sleet/Ice Pellets (PL): are frozen raindrops that strike the earth's surface. In a sleet situation the
precipitation aloft when it is first generated will be snow. The snow falls through a layer that is a little
above freezing and the snow partially melts. If the snow completely melts it will be more likely to
reach the earth's surface as supercooled water instead of sleet. If the snow partially melts there will
still be ice within the falling drop for water to freeze on when the drop falls into a subfreezing layer.
The lowest layer of the troposphere will be below freezing in a sleet situation and deep enough to
freeze drops completely. The lower boundary layer can be above freezing and sleet occur if the sleet
does not have time to melt before reaching the surface.
7. Hail (GR) - Hail is dense precipitation ice that is that least 5 millimeters in diameter. It forms due
to ice crystals and supercooled water that freeze or stick to the embryo hail stone. Soft hail is more
white and less dense since it has air bubbles. Soft hail occurs when hail grows at a temperature below
freezing by ice crystals and small supercooled water and cloud droplets merging onto the hail. Hard
hail occurs when liquid water drops freeze on the outer edges of the hailstone after the outer edge is
above freezing. The freezing of supercooled water releases latent heat and this can result in the outer
edge of the hail stone warming above freezing. Then the water refreezes creating solid ice. Hail will
commonly have soft ice and hard ice layers when it is sliced open.
8. Graupel (GS) - Graupel forms in the same way as hail except the diameter is less than 5
millimeters. It usually grows by soft hail processes.
9. Drizzle (DZ): Drizzle is liquid precipitation that reaches the surface in the form of drops that are
less than 0.5 millimeters in diameter.
10. Freezing Drizzle (FZDZ): Freezing Drizzle is liquid precipitation that reaches the surface in the
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form of drops that are less than 0.5 millimeters in diameter. The drops then freeze on the earth's
surface.
11. Freezing Rain (FZRA): Freezing Rain is liquid precipitation that reaches the surface in the form
of drops that are greater than 0.5 millimeters in diameter. The drops then freeze on the earth's surface.
12. Freezing Fog (FZFG): Freezing fog is a fog composed of supercooled water drops. These drops
freeze just after they wet the earth's surface.
13. Mixed Precipitation (MXD PCPN)- The combination of two or more winter precipitation types
occurring at the same time or over a period of time at the same place.
Physical processes:
1. Microphysical processes

Warm cloud (T > 0ºC) in tropical grows by collision-coalescence (合并)

Cold cloud (T< 0ºC) in mid-latitude. The upper and mid levels (-15 to -20°C)
supply ice crystal via depositional growth (maximized at -15°C). Aggregation (聚合),
rime (结晶) splintering (分裂), and accretion (增长) occur most rapidly at the lower
levels of the cloud between 0°C and -10°C. The lowest level is the splintering and
accretion process.

Seeder-Feeder Mechanism.


This occurs when a cloud with ice crystals (seeder) at a given T moves over the
top of a lower supercooled liquid cloud and precipitates ice down into that cloud
(feeder).
the seeder (ice) cloud must not be more than 5000 feet above the supercooled
liquid cloud (feeder)
2. The vertical thermal structure of the atmosphere.
LIQUID: Deep surface layer with temperature above zero.
FREEZING: Surface temperature below zero with an above freezing layer.
FROZEN: Deep surface base layer with temperatures below zero
PCPN Forecasting Strategies
1. NWP (vertical motion ?): 250 mb jet streak; 500 mb PVA; 700 mb vertical
velocity progs and RH panel; 850 mb PTA
2. Tephi: convective precipitation and precipitation type. Pay attention to
moist/dry layers, wind profile. Modify Tephi as necessary.
3. Examine MOS precipitation data
4. Look at satellite, radar (bright band), how they will move through time?
5. Correlating current and/or past observations of precipitation type
boundaries with thermal fields such as thickness and by predicting the
short term evolution of the correlated thermal field
6. Decide on characteristics of PCPN: Convective or stratiform? Heavy or
light? widespread or numerous? Severe or non-severe? Long lasting or
brief?
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RAIN (radii > 0.5 mm)

warm rain produced by warm
cloud several km in depth

the wet-bulb temperature is above
0°C

Rain from ice crystals:
snowflakes or graupel in the subfreezing layers of the cloud melt
and fall to ground.
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DRIZZLE (diameter ~0.1 mm)


Falls from stratus cloud 1km or less in
depth.
Cloud-top T < -10 C


strong cooling by the underlying
surface,
gradual upslope flow providing
sufficient lift.
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SNOW
Large snowflakes at T near freezing while small flakes at colder T

Is the surface temperature < 1.7°C ?

Is the freezing level < 1200ft (366m)?
Note: If the freezing level is > 1200ft,
mixed precipitation is likely.

Is the 850 mb temp < 0°C?

Is the 700 mb temp <-4°C?

Is the 1000-500mh thickness < 5400?

Is the temperature <0°C from 1200ft
to 700 mb?

Is there a moist layer (T-T depression
d
5°C) from surface to 700 mb?
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FREEZING RAIN

Super cooled rain droplets that freeze upon contact with the ground or objects

An above freezing layer exists which is at least 30 mb.

A below freezing layer is present (about 1500 feet thick).

Surface T around 0°C.

There is a low concentration of ice nuclei in the sub-freezing layer.

Alert area between zero line in 850mb and surface warm front
ICE PELLET (Diameter 2-7 mm)

form in a similar manner to freezing rain.

If the below-freezing layer is sufficiently deep the supercooled raindrops will
freeze into ice pellets before they reach the ground.

Ice pellets often precede or accompany freezing rain and with ice pellets there is
freezing rain aloft
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HAIL (Diameter 5-30 mm)
 The surface temperature of graupel particles is less than 0 C while for hailstones it
is approximately 0 C.
 High liquid water content in the cloud and strong updrafts.
 Hail usually falls from CB clouds, small hail may develop in TCU.
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FREEZING DRIZZLE
“non-classical” (and most prevalent) mechanism for formation of freezing drizzle is
through the collision-coalescence process
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Entire sounding below freezing 
At 700 mb the flow tends to be
15 to 0°C.
light to moderate cyclonic, and

An inversion with a deep dry
the relative humidity is 30-50%
layer above the inversion and a
or less.
shallow moist layer below the
inversion.

Significant wind shear near the
inversion level

The wind speeds 5 knots or more.

The cloud bases under 4000 feet,
with the best cases under 1000
feet.

Cloud thicknesses are 2000 to
6000 feet

The air is often saturated though
850 or 800 mbs.
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SNOW GRAIN (SG, diameter < 1 mm)

Solid equivalent of drizzle

Very small white & opaque
grains of ice

occur at colder T than freezing
drizzle

Fall from low, thin stratus clouds;

grow by diffusion and
aggregation.

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ICE CRYSTALS (Diameter < 1 mm)
 Occurs at the surface falling from either clear or cloudy skies, usually when the
temperatures are very cold.
 The crystals grow by diffusion of water vapour primarily onto the sharp corners
and edges.
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